If you want to change your magento theme with mobile platform . It`s very simple just do it what i tell you in following. In Magento Admin : System > Configuration > Design is where you want to be. You also want to make sure you have the mobile designed theme . Configuration the following way Depending on how you want to serve up your store depends on what configuration you want.

Option #1 :

Show mobile version on Any Mobile Based OS (phone ,tablet, kindle fire)

iPhone|iPod|BlackBerry|Pre|Palm|Googlebot-Mobile|mobi|Safari Mobile|Windows Mobile|Android|Opera Mini|mobile|Silk|Kindle

Option #2:

Show mobile version to just phone based browser and Display normal desktop version on tablet based devices

(iPhone|iPod|BlackBerry|Pre|Palm|Googlebot-Mobile|mobi|Safari Mobile|Windows Mobile|Android|Opera Mini|mobile)|(!iPad|Silk|Kindle|Xoom|SCH-I800|Tablet)  


You have chosen one of the configurations above .Go under the Design “Themes” section & add it to the following sections (Template, Skin, Layout, Default) in the “Matched Expression” field.  Then in the values section you should put in “iphone” in the value field. If you have a different mobile theme then your value field should be the theme name. Then click save config. reefer this following screenshot. mobile theme for magento.